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1. Technical Field of the Invention
This invention relates to high-efficiency ( greater than 99.5%) power conditioning electronics used to convert the DC output of fuel cells into suitable AC power, especially useful for applications in vehicles such as buses, trucks, and ships.
2. Description of Related Art
Prior art is given by the existing technologies for transportation based on combustion engines. A worldwide effort is underway to implement new, environmentally cleaner means of portation by converting to electric propulsion. Fuel cells are considered for this application.
According to news reports [1] General Motors developed a car called xe2x80x9cZafiraxe2x80x9d which operates on fuel cells powered by liquid hydrogen, which has a cryogenic temperature of 20.27 K (xe2x88x92252.88xc2x0 C.). Other fuel cells use a reformer which extracts hydrogen gas from natural gas (methane, CH4) or gasoline [3, 4]. Alternatively, liquid natural gas (LNG) at a temperature of xe2x88x92161.5xc2x0 C. (112 K) presents interesting possibilities which will be part of this invention disclosure.
Fuel cells deliver DC power, which is then converted by so-called inverters into AC power required for general use and for the efficient operation of electrical motors. These inverters are often larger, heavier, and more expensive than the motors they control.
The new concept of Cryogenic Energy/Power Conversion (CEC, CPC) has achieved drastic reductions in size, weight, and cost in the field of power electronics (Cryo-Micro-Power, CMP). Such size and weight reductions are nowhere more important than in vehicles, where energy savings are crucial. However, when applied to motor drives for transportation (or other) applications, CEC presents a major problem: the cooling, which adds further weight and requires an additional tank. This problem is solved in the case where a cryogenic fuel such as liquid hydrogen or liquid natural gas are already available, opening up interesting possibilities. On the other hand, CEC can achieve its full potential only if implemented in the form of Cryo-Multichip Modules (CMCM) made with the wireless High-Density Interconnect (HDI) technology [P6,11,12,13]. This technology interconnects power transistor/diode chips in a half-bridge or full-bridge topology with polymer and metallic thin-films, thus eliminating the weak link in power electronics: the wire bond connections. Wireless interconnection provides many advantages such as higher switching speeds, higher frequencies, and improved efficiency and reliability.
The great potential of silicon applications in power and energy conversion (solid-state transformers, inverters, etc.) has not yet been adequately addressed by the semiconductor industry. It is desirable to change this situation by promoting the concept of cryogenic energy conversion, discussed in greater detail below. Furthermore, CMCMs are most important in transportation systems, and will be the key component for realizing Cryogenic Energy Conversion. An application example is described in the following pages. Systems can be optimized by combining CMCMs with High-Temperature Superconductors, especially when used to implement small filter inductors.
The widespread application of electric vehicles requires the development of a sufficiently small, light, and efficient motor drive or Adjustable Speed Drive (ASD) to couple the fuel cell or battery output to the motor. Therefore multichip modules are proposed for an efficient motor drive system based on the following assumptions and suggestions:
Sooner or later, High-Temperature Superconductors (HTS) will be commercially available for applications in the power and energy generation and distribution fields at competitive prices: HTS cables, HTS transformers, HTS motors, HTS generators, etc. Billions of dollars have already been invested in this new HTS technology since its discovery in 1986.
HTS components require Cryogenic Cooling. In most cases, such as in HTS cables and transformers, liquid nitrogen (LN2), at a temperature of 77 K (xe2x88x92196 C), will be used
The availability of HTS components requires a rethinking and redesigning of many energy systems. HTS Technology can best be supported by the new concept of Cryogenic Energy Conversion (CEC) based on Low Temperature Electronics (LTE) and Cryo-MOSFEts, Cryo-IGBTs, or other cryogenically operated devices.
CEC can provide a considerable improvement in power and energy conversion efficiency as well as a drastic reductions in size, weight and, therefore, cost: Micro Cryo-Power. CEC represents the mating of High-Temperature Superconductors with Low-Temperature-operated Semiconductors. Electronic efficiencies of  greater than 99.5% should be possible (not considering the cooling penalty).
The efficiency of electrical motor operation can be drastically enhanced by applying CEC to Motor Drives or ASDs. CEC would miniaturize these drives, which can be 2-3 times more expensive in prior art technologies, and are also much larger and heavier than the motors they control.
Size and weight reduction, along with improved conversion efficiency, is nowhere more important than in transportation vehicles. Every kilogram of weight reduction translates into a considerable energy saving for vehicles traveling hundreds of thousands of miles in a lifetime.
The push for higher efficiency leads to a push for electric vehicles requiring motors and ASDs.
Great progress has been made recently (New York Times, Oct. 21, 1997) in the field of Fuel Cells using gasoline or Liquid Natural Gas (LNG: 112 K, xe2x88x92161 C).
Therefore, this invention describes an ultra-small and light-weight Cryogenic Adjustable Speed Motor Drive in the power range of 50 to 200 Hp (35 to 150 kVA) using Cryo-MOSFETs or other suitable devices such as IGBTs.
Tremendous commercialization opportunities providing higher energy conversion efficiencies can be envisioned for many transportation systems combining (H2, O2) fuel cells using LNG, LH2, HTS motors, HTS cables and Cryo-Motor-Drives.
In the case where HTS motors are used, the small cryopower electronics can be integrated inside the HTS motor or onto the case of the HTS motor.
Motor drives using Cryo-Multichip Modules (CMCM) arc intended for application in vehicles (buses, trucks, trains, ships, airplanes) as one important component in the coming age of Cryogenics which will combine High-Temperature SUPER-Conductors with Low-Temperature SEMI-Conductors. Such Adjustable Speed Drives (ASDs) will, of course, find applications in stationary systems as well. Many manufacturing plants requiring ASDs already use liquid nitrogen for other purposes. The proposed CMD will provide smaller size, reduced weight and increased efficiency due to its application of the new concept of Cryogenic Energy Conversion (CEC). Every kilogram of weight reduction translates into a considerable fuel saving over the lifetime of a vehicle running hundreds of thousands of miles. Such a development should be desirable in view of the fact that the federal government now mandates that cities of certain sizes must provide alternatively-fueled methods of public transportation (xe2x80x9cCold Factsxe2x80x9d, Summer 96 Issue).
Nothing beats semiconductor technology as far as reliability and reductions in size, weight, and cost are concerned. It is finally time to apply this technology to the field of (high) power conversion. This is made possible by the concept of Cryogenic Energy Conversion (CEC).
A press release of Aug. 1, 1997 reads as follows: xe2x80x9cGovernor Pataki Announces Bond Act Funding for Clean Busesxe2x80x9d. Also: xe2x80x9cGovernor George E. Pataki today announced the State will award $3 million for the purchase of 39 clean-fuel buses as part of the Clean Fuel Bus Program under the Clean Water/Clean Air Bond Actxe2x80x9d. These hybrid buses use CNG (compressed natural gas) to fuel a Diesel engine. They are described in an article by King et al., in the IEEE Spectrum of July 1995 [6] which presents the prior art. His figures [6, p. 29] show how small the motor is compared to the inverter motor drive.
The next step in the development of the electric transit bus could be the use of fuel cells to replace the diesel engine as a power source. Many companies are working to develop such fuel cells. Very interesting possibilities exist in the application of the new fuel cell technology. For large vehicles such as buses, LNG (liquid natural gas) is desirable, and makes the task of applying fuel cell technology easier. CVI, Inc. has successfully designed and manufactured onboard LNG fuel systems for buses such as those used by the Houston, Tex., METRO transit coach system.